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Respiratory & Heart Disease in Rats

This information is from my booklet Rat Health Care. I highly recommend you order a copy to have on hand! Check it out at Rat Books

Respiratory disease is the most common health problem and cause of death in pet rats. The most common organism causing this disease in rats is the bacteria Mycoplasma pulmonis, which can enter the body’s cells in order to hide from antibiotics. While antibiotic treatment can help control the disease, there is no known cure for rats.

Mycoplasmosis is extremely contagious and baby rats contract the bacteria from their mother during birth. The disease has become so common that pretty much all pet rats are infected, whether they have symptoms or not. Laboratory rats are free of the disease because in the past, baby rats were delivered by Caesarian section while the mother was submerged in disinfectant, sacrificing the mother. All laboratory rats in the world are now descended from these hand-fed babies.

The Mycoplasma bacteria live in the lungs, therefore it is incorrect to refer to “upper respiratory infections (URI)” in rats. Mycoplasma commonly causes pneumonia, lung abscesses, emphysema and lung lesions which on autopsy can appear as a “cobblestone” effect on the surface of the lungs. It can also cause inflammation and bleeding of the uterus. To see autopsy pictures of rat lungs click here.

A mycoplasma infection makes a rat more susceptible to secondary respiratory bacterial infections as well. The respiratory symptoms that are common in rats can be caused either by the primary mycoplasma infection, secondary infections, or both as well as heart disease. In young rats, symptoms are more likely to be caused by secondary infection, so they need to be treated accordingly. To see autopsy pictures of rat hearts, click here.

Different rats also seem to have differing resistance to the disease. The severity of a mycoplasma infection can be increased by cigarette smoke, ammonia from a dirty cage, vitamin A or E deficiency, pine or cedar shavings, and a concurrent respiratory infection of another type, as well as genetic susceptibility.

(None of the organisms causing respiratory infections in rats are infectious to humans. The only diseases I know of that can be transmitted from domestic rats to humans are salmonella and “rat-bite fever,” a rare bacterial infection similar to cat-scratch fever. For more info see my article at

Common Symptoms
The first symptom of a respiratory infection is usually frequent sneezing (healthy rats rarely sneeze) which can progress to wheezing. To hear examples of rats wheezing, click here. Often, the first symptoms of respiratory disease seen in a rat, especially in young rats, will be caused by a secondary infection. Mycoplasmosis tends to be a slowly progressive chronic disease that usually doesn’t cause symptoms until the rat is older than 8 months of age.

For any symptoms, I recommend first trying amoxicillin, which tends to work well for secondary infections. Secondary infections can become more serious more quickly than mycoplasma, so treat for them first. If you treat for mycoplasma first, and it turns out to be a secondary infection, the rat may get so sick he dies before you can try the treatment for secondary infections. Usually a mycoplasma infection is chronic, that is, it starts out slowly with mild symptoms that gradually get worse over a period of weeks or months, so quick treatment for myco is not as critical as it is for a secondary infection. I recommend that all rats owners have amoxicillin on hand so sick rats can be treated ASAP.